1. Field of the Invention
The present invention relates to a technique of soldering electronic components such as IC and LSI to a printed circuit board using a laser beam.
2. Description of the Related Art
The technique of soldering the electronic components such as IC and LSI to the printed circuit board using the laser beam is well known (for example, see Japanese patent Application Laid-open Nos. 4-52073 and 2002-1521). In the technique, the printed circuit board is irradiated with the laser beam from a soldering head of a soldering apparatus, and the solder is melted by heat of the laser beam to perform the soldering. Advantageously the soldering can be performed in a non-contact manner.
However, as illustrated in FIG. 7, when a rod terminal 4a of an electronic component 4 is inserted in a through-hole 3 surrounded by a ring-like terminal 2 of a printed circuit board 1 in order to solder the rod terminal 4a and the ring-like terminal 2 using wire solder 5, the laser beam 6 is projected onto the electronic component 4 through a gap 3a between the ring-like terminal 2 and the rod terminal 4a before the wire solder 5 is melted by a laser beam 6 and the through-hole 3 is embedded, which results in a problem in that burning is generated in part of the electronic component 4. The wire solder 5 contains flux in a cavity portion located in the center thereof, and a flux content is as low as about 3% (ratio by weight) because of a structure of the wire solder 5. Therefore, due to wettability after melting, unfortunately it takes a long time for the melted solder to spread along the ring-like terminal 2 or the rod terminal 4a or to flow in the through-hole 3.
On the other hand, solder paste in which solder grains are formed into a paste is used instead of the wire solder 5, the solder paste is previously fed to the ring-like terminal 2 and the rod terminal 4a so as to fill the through-hole 3, and the solder paste is irradiated with the laser beam and melted to perform the soldering. In such cases, the laser beam is blocked by the solder paste not to pass through the through-hole, so that the electronic component burning problem can be avoided.
However, because the solder paste has fluidity, when an amount of solder paste necessary for the soldering between the ring-like terminal and the rod terminal is fed in a stack manner onto the ring-like terminal, the solder paste is diffused to flow to the outside of the ring-like terminal until the solder paste is irradiated with the laser beam. At this point, 10 when the solder paste is irradiated with the laser beam, the solder paste located outside the ring-like terminal is left while not melted, and the solder paste is scattered around, which easily causes a poor appearance and a conduction defect. Further, because the large amount of solder paste is fed, not only it takes a long time for the solder paste to be melted with the laser beam, but also the granular paste in the solder paste flowing previously in the through-hole is not completely melted to leave the granular paste. Therefore, a soldering defect is frequently generated.